docs: PRD + plan to move FruitCake "Watch AI" to the client (M30)

[PieterjanDeClippel]

What

Adds docs/prd/FRUITCAKE_CLIENT_AI_PRD.md and the matching M30 milestone entry in PLAN.md. Docs only — no code changes.

Why

FruitCake's "Watch AI" runs entirely server-side today: a per-connection WebSocket loop runs the C# physics + DuelingQNet + depth-3 search, streaming ~30 fps frames (FruitCakeController.Live). On the CPU-only Hetzner VPS that's a persistent per-viewer CPU cost with no admission control — N viewers saturate the cores and frame deadlines slip for everyone.

This PRD moves the whole agent into the browser (net inference, observation, physics, search), collapsing the server to static-asset + checkpoint hosting. Per-viewer standing CPU → ~0.

Key points

• Deliberately reverses the original server-authoritative serving decision (FRUITCAKE_AI_PRD.md §4.6/§4.8) — the target shifted from "ship correctly" to "serve cheaply at concurrency."
• Central de-risk: moving the search client-side makes C#↔TS physics parity load-bearing. The plan builds the cross-language golden-vector conformance test that §4.8 recommended but never shipped, and ships on same-column-choice parity, not bit-equality.
• Server engine preserved (owner's ask): FruitCakeWorld/Env/Search/Controller stay for training + as the parity oracle; the WS just leaves the serving hot path (optionally flag-gated).
• Plan G0–G7: TS net + /api/fruitcake/weights → TS observation → TS physics search surface → conformance test → TS search → Web Worker + UI cutover → ≥200-game client-vs-server validation → (stretch) generalize to Snake/MountainCar.

🤖 Generated with Claude Code

[PieterjanDeClippel]

Superseded — closing. This docs-only PRD proposed moving FruitCake's "Watch AI" into the browser; that's now shipped:

• FruitCake client-side AI — FruitCake: single-source physics + AI via Polyglot → fully client-side AI (zero server inference); + NetTransfer SDK util #23 (M32, merged). Implemented via docs/prd/FRUITCAKE_CLIENT_SIDE_AI_PRD.md: the observation + net forward + depth-3 search single-sourced in fruitcake_solver.pg (byte-identical C#↔TS via MintPlayer.Polyglot), checkpoint shipped to the browser, server serving path retired.
• Snake + MountainCar client-side AI — M33: Snake & MountainCar AI run fully in the browser (zero server inference) #24 (M33, merged). This PR's stretch goal (generalize to the other WebSocket-AI games) — same single-source .pg pattern; EpisodeStreamer + app.UseWebSockets() removed, so no server-side inference/WebSocket remains anywhere.

The delivered approach differs from this draft in the good direction: rather than a bespoke TS net + /api/fruitcake/weights + hand-maintained golden-vector parity, the whole inference path is single-sourced through Polyglot (f64 both sides ⇒ parity by construction). The FRUITCAKE_CLIENT_AI_PRD.md doc / "M30" label here would also collide with the shipped M30 (big-fruit inputs). No code from this branch is needed.